The theme continues to be the identification of factors that govern the efficiency of ATP synthesis, utilization, and the maintenance of ion gradients in the myocardium. The interconversion between chemical and ion gradient energy is very highly coupled in myocardial cells, thus imbalances between energy supply and demand are unforgiving. The rationale is to provide novel physiological concepts which are at the core of understanding the development and prevention of myocardial disease occurring from insufficient chemical energy supply (ischemic heart), imbalance between ATP production and utilization (hypertrophy heart), or derangement of signaling or ion gradients (arrhythmia). This Program consists of 4 components. The titles are: Novel Regulation of Cellular Mg2+ and Ca2+ Homeostasis in the Heart. Release and Metabolism of Extracellular ATP in the Heart: Crosstalk between Cardiac Bioenergetics and Signal Transduction. Thyroidal Regulation of Na+, K+-ATPase Expression and Energetics of the Heart. Time-resolved X-ray Interferometry Studies of the Sarcoplasmic Reticulum Calcium ATPase. Some of the broad integrated questions to be addressed in this application are: how does catecholamine-induced redistribution of Mg2+ between tissue and plasma modulate ATPases and cellular ion content?; how do extracellular ATP molecules and Mg2+ produce signals in the heart which result in adenosine formation, PKC signaling and redistribution of ions?"; how does thyroid hormone, through specific regulation of the Na+-K+ ATPase expression and creatine transport increase ATP synthesis and utilization while decreasing the maximal energetic capacity of the myocardium?; how, through the brightest x-ray sources coupled with a novel detection system, it is possible to correlate kinetically profiled structures of the various intermediate states of the SR Ca2+ ATPase and Ca2+ location during catalysis?. The major strength of this Program is the convergence of important aspects of myocardial function by several established and interactive researchers, bringing to the theme a broad repertoire of unique expertise. A major revision occurred in Projects 1 and 3, while Projects 2 and 4 have been nearly entirely rewritten.